Nitric acid and the origin and size segregation of aerosol nitrate aloft during BRACE 2002

As part of the BRACE 2002 May field intensive, the NOAA Twin Otter flew 21 missions over terrestrial, marine, and mixed terrestrial and marine sites in the greater Tampa, Florida, airshed including over Tampa Bay and the Gulf of Mexico. Aerosols were collected with filter packs and their inorganic fractions analyzed post hoc with ion chromatography. Anion mass dominated both the fine- (particle diameters 2.5 μm) and coarse-mode (particle diameters 10.0–2.5 μm) inorganic fractions: SO42−in the fine fraction, 3.7 μg m−3 on average and Cl− and NO3− in the coarse fraction, 0.6 μg m−3 on average and 1.4 μg m−3 on average, respectively. Ammonium ion dominated the inorganic fine-mode cation mass, averaging 1.2 μg m−3, presumably in association with SO42. Coarse-mode cation mass was dominated by Na+, but the concentrations of Ca2+ and K+ together often equaled or exceeded the Na+ mass which was, on average, 0.6 μg m−3. Nitrate appeared predominantly in the coarse rather than the fine fraction, as expected, and the fine fraction never contributed >15% of the total NO3 concentration. Nitric acid dominated the NO3− contribution from both aerosol size fractions, and constituted at least 45% of the total NO3 in all samples. Coarse-mode Cl− depletion, and hence NO3− replacement, reached 100% within the first 4 h of plume travel from the urban core in some samples, although it was most often less than 100% and slightly below the expected 1:1 ratio with coarse-mode NO3− concentration: the slope of the regression line of NO3− concentration to Cl− depletion was 0.9 in the coarse fraction. In addition, terrestrial samples were markedly lower in Cl− depletion, and thus in substituted NO3–, than were marine and mixed samples: 15–25% depletion in terrestrial samples vs. 50–65% in marine samples with the same air mass age. Thus, we conclude that NO3− and its progenitor compound HNO3 were present in the Tampa airshed in insufficient amounts to titrate fully the slightly alkaline coarse-mode particles there, and to replace completely the Cl− from the coarse-mode NaCl.